1. Field of the Invention
The present invention relates to the synthesis of acyl cyanides, and, more especially, to the synthesis of acyl cyanides by reacting acid halides with an alkali metal cyanide.
2. Description of the Prior Art
The acyl cyanides are known intermediates for a variety of organic syntheses, for example the production of herbicides.
FR 2,353,524 describes a synthesis of benzoyl cyanide C.sub.6 H.sub.5 COCN by reacting benzoyl chloride with a molar excess of sodium cyanide in the presence of a nitrile of a carboxylic acid and copper cyanide.
FR 2,346,323 describes a similar, but much more general reaction, since it is applicable to an entire class of acyl cyanides, and entails reacting sodium cyanide with an excess of acid halide in the presence of copper or zinc cyanide.
The above processes present the disadvantage of requiring the presence of heavy metals and therefore mandate complicated treatments to avoid their presence in the reaction effluents.
Tetrahedron Letters (Pergamon Press) No. 26, pages 2275-2278 (1974) describes a process limited to the synthesis of benzoyl cyanide by reacting sodium cyanide with benzoyl chloride in solution in methylene chloride and in the presence of tetrabutylammonium bromide; the C.sub.6 H.sub.5 COCN yield based on C.sub.6 H.sub.5 COCl converted does not exceed 60%.
FR 2,364,894 describes the synthesis of C.sub.6 H.sub.5 COCN by reacting C.sub.6 H.sub.5 COCl with NaCN in a solvent in the presence of benzoic anhydride (C.sub.6 H.sub.5 CO--O--CO--C.sub.6 H.sub.5) or of products which can generate benzoic anhydride under the reaction conditions. Example 1 of this patent was reproduced, namely, the reaction of benzoyl chloride, benzoic anhydride and sodium cyanide in xylene, at a temperature ranging from 140.degree. to 145.degree. C. for 8 hours, and a 51.2% molar yield of benzoyl cyanide was attained, based on the combined benzoic anhydride and benzoyl chloride converted, and a degree of conversion of 60.5%, based on the benzoyl chloride consumed. This examples was again repeated, except that moist sodium cyanide (0.4 g of water per 36.75 g of NaCN) was used; the yield increased from 51.2% to 88.2%.
Example 2 was also reproduced, namely, the reaction of benzoyl chloride, sodium cyanide and sodium benzoate in xylene at 135.degree. C. The benzoate is presented to generate benzoic anhydride by reaction with benzoyl chloride. A degree of conversion of 23% of the benzoyl chloride consumed was determined and a 14.9% molar yield of benzoyl cyanide, relative to the benzoyl chloride consumed, instead of the reported 94%. This Example 2 was again repeated, except that moist sodium cyanide (0.5 g of water per 29.4 g of NaCN) was used; the degree of conversion increased from 23% to 85.7% and the yield from 14.9% to 67.8%.
The presence of water is therefore necessary to obtain an economically acceptable yield according to FR 2,364,894.
Example 3 of FR 2,364,894 relates to reacting benzoyl chloride, sodium cyanide and water in xylene at 135.degree. C.
This is similar to Example 2, but the benzoate precursor of the anhydride is replaced with water. This Example 3 was reproduced and, after 2 hours of heating to 135.degree. C., the yield did not exceed 60%.